string hello = "Hello World!";
Replace(hello, "o", "_");
cout << hello << endl;
выведет: Hell_ W_rld!

и ещё

string hello = "Hello Worlld!";
Replace(hello, "ll", "[*]");
cout << hello << endl;
выведет: He[*]o Wor[*]d!

#include <iostream>
#include <string>

using namespace std;

void Replace (string& source, string find_what, string replace_with) {
    if (source == "" || find_what == "") {
        return;
    }
    if (source == find_what) {
        source = replace_with;
        return;
    }
    unsigned int findwhat_index = 0,
    start_index = 0,
    find_length = find_what.length(),
    flag = 0;
    for (unsigned int i = 0; i < source.length(); i++ ) {
        switch (flag) {
            case 0:
                if (source.at(i) == find_what.at(0) && find_length == 1) {
                    source = source.replace(i, 1, replace_with);
                }
                else if (source.at(i) == find_what.at(0)) {
                    start_index = i;
                    findwhat_index++;
                    flag = 1;
                }
                break;
            case 1:
                if (source.at(i) == find_what.at(findwhat_index) && findwhat_index < (find_length - 1)) {
                    findwhat_index++;
                }
                else if (source.at(i) == find_what.at(findwhat_index) && findwhat_index == (find_length - 1)) {
                    source = source.replace(start_index, find_length, replace_with);
                    findwhat_index = 0;
                    start_index = 0;
                    flag = 0;
                }
                else {
                    findwhat_index = 0;
                    start_index = 0;
                    flag = 0;
                }
                break;
        }
    }
}